The present invention relates to a process for the preparation of tubular chills (also called ingot moulds) of copper or copper alloys of the type shaped in such a way as to have a substantially curved longitudinal axis and intended for installations for the continuous casting of steel.
In an installation for the continuous casting of steel, as is known, the said chills are traversed by a flow of molten metal, which commences its solidification whilst traversing these, under the action of a vigorous cooling achieved by the circulation of a coolant fluid which flows over the outer surface of the chills themselves.
In order to perform effectively the functions which are required of them chills of this type must have a complex set of favourable properties. First of all they must be provided with internal surfaces having a very high surface hardness and a finish such as to allow the deposition thereon of a layer of cladding material (for example chrome) able to resist effectively the wear action caused by the sliding of the molten steel, as well as allowing this sliding to take place with low friction; in the second place, the cross-section of the chill must decrease slightly along its axis (conical profile) in such a way as to ensure always a perfect transmission of heat towards the coolant means which flow over the outer surface of the chill: it has in fact been found that, whenever such a reduction in cross-section along the axis is not present, because of the shrinkage of the material which solidifies in correspondence with its outer layers, it is possible for the metal to become separated from the internal surface of the chill which considerably reduces the coefficient of transmission of heat between the metal and the chill itself.
For the preparation of chills of the above-indicated type it is normal to start with a blank of tubular form having a rectilinear axis obtained simply by drawing or by any other operation. Subsequently a curved form is imparted to it, normally by exerting radial pressures on its outer surface by means of a die of suitable form; subsequently, for the purpose of creating the desired surface finish and the variation in cross-section along the axis necessary for achieving the correct flow of steel along the chill itself, the internal surface of the curved blank is worked by material-removing operations such as grinding or lapping, or else in an alternative process by progressive chemical attack differentiated in depth along the axis of the blank. Chills obtained in the manner described have numerous disadvantages consisting principally in a poor surface durability of the internal surface of the chill and in a poor surface finish thereof; moreover, both the mechanical working and the process of chemical attack necessary to achieve the conical internal surface of the ingot mould are rather lengthy and complex operations.
For the purpose of overcoming these disadvantages Italian Patent No 1,160,132 in the name of the same applicant describes a process in which an ingot mould or chill is formed to the required size using only plastic deformation operations thanks to the introduction, within a previously bent blank, of a curved mandrel having externally the final form of the ingot mould which it is desired to obtain and a subsequent drawing operation effected on the assembly of elements constituted by the blank with the said curved mandrel housed within it.
However, even this process is not entirely free of disadvantages; in particular, especially when it is necessary to obtain ingot moulds of rather great length, this known process does not allow the dimensional precision and required form to be obtained both because of the difficulty of centring the mandrel within the curved blank and because of problems of elastic instability of the blank during a stage before the drawing stage and in which the blank is provided with an axial shoulder which subsequently serves as an abutment for the mandrel during the drawing stage.